1. Field of the Invention
The present invention relates generally to optical coupling of a high brightness light source to a plurality of optical light conductors and, more particularly, to an optical coupling assembly providing an increased number of internal reflections and thereby improved light color mixing.
2. Description of the Prior Art
Central lighting systems wherein a single light source is used to supply light output to a number of spaced apart locations using a plurality of optical light conductors, such as optical tubes, rods or fibers, have been proposed for a number of different applications including automotive, display and home lighting. For instance, U.S. Pat. No. 4,958,263 issued to Davenport et al and assigned to the same assignee as the present invention discloses the use of a central lighting system for an automotive application.
The light source for such a central lighting system must be capable of providing a light output having high brightness characteristics so as to ensure sufficient light output at the spaced apart locations. One suitable light source having a high brightness characteristic measuring in excess of 50,000 lumens per square centimeter is disclosed in the second patent application cross-referenced above. Such a light source when used in conjunction with a suitable reflector, such as one having an ellipsoidal shape, and a mirror element to focus the light output on the ends of optical fibers disposed at an optical focal plane of the reflector can produce a non-coherent high brightness focussed light output capable of causing physical damage to the ends of the optical fibers. To avoid the possible damage to the ends of the optical fibers, a light guide in the form of an one-piece optical cylindrical coupler rod, preferably about 11/2 inches in length and 3/4 inch in diameter, is interposed between the optical focal point and the ends of the optical fibers. The light output travels through multiple internal reflections from the optical focal point to the optical fibers.
Also, the non-coherent high brightness light output image produced by the light source and focussed by the ellipsoidal reflector tends to be brightest and more blue in color at the center of the light output and dimmer and more red in color around the outer edge of the light output. When the light output enters the optical cylindrical coupler rod, the multiple internal reflections tend to achieve some mixing of the light and thereby reducing the differences in color from the outer edge to the center of the light output entering the optical fibers.
In order to achieve adequate light mixing, some minimum length to diameter ratio is needed for the coupler rod. The inventors herein have found that a length of 11/2 inches for a coupler rod of 3/4 inch diameter, or a ratio of 2, is not adequate to produce sufficient light mixing to substantially reduce the color differences. A cylindrical coupler rod of sufficient length to provide adequate light mixing would be too long for the central lighting system in the automotive lighting application. Therefore, a need remains for improvement of the design of optical coupling rods so as to increase light mixing without increasing substantially the length of the rod.